Steve - the temp sensor voltage and water temp cross around 200-215 deg = 2.00 - 2.15 volts As the temp goes up the volts go down.
There is no reflash for this concern. Most - if not all flashes deal with reporting false check engine lights and tweaking air fuel mixtures.
You should see about 4.98 volt measured or read on the scanner with the sensor unplugged. Looking at the wiring diagram - the sensor -coolant - is provided its own 5 volt signal and shares a common sensor ground. Its possible that the pcm is not able to supply the proper voltage and current to provide the correct voltage drop that is needed to signal a fan request- very rare . That would set a fault over time.
If your scanner can see fan request and also can actuate the fan relay - try actuating the cooling fans . turn the a/c on and monitor the a/c pressure and request for cooing fan from the pcm . I checked for the temp vs volts table- but if think thats listed in one of the student training books - that we all loose or toss out! .
You should have 50% fan speed at 220 deg and shuts off at 214 deg- The fan relay is electronic and controlled by a pulse width 12 volt signal from the PCM .
Let me know if you can control the fan relay with your scan tool. and if the A/C will turn the relay on- You may have a failing relay .
The radiator may be plugged up and not allowing coolant flow , and this would cause the
overheat condition also- I am not sure if you can get a look at the inlet side of the radiator to inspect for plugging.
Seems you have checked every angle . You would have to use a cooling system flow tester connected to the rad outlet side. You should also be able to feel or shoot a temp readings across the radiator when the t- stat opens. Cool areas will be blockage .
Hope this helps . Please reply if you need more help or if clarifications . Thanks Scott